Field of the Invention
Increasing the number of levels of interconnects in integrated circuits provides additional routing capability, more compact layouts, better circuit performance and greater use of circuit design within a given integrated circuit surface area. A particularly useful level of connection is commonly called local interconnection where neighboring diffused areas are connected to one another and to neighboring polysilicon and metal lines.
A conventional method for creating local interconnects uses metal interconnection of diffused regions to one another as well as to other layers. The metal interconnection is formed by etching vias through a thick oxide layer to the locations to be interconnected. A conductor is then formed to fill the vias and make the connection. This method is limited, for purposes of reducing the area required for such connection by the state of the technology of etching contact holes and the planarization of interlevel dielectrics. These limitations include the alignment tolerance of the vias to the underlying region to be connected, the size of the via required (and accordingly the size of the contact area in the underlying region) which can be reliably etched and the step coverage of the conductor filling the via and making good ohmic contact to the underlying region. Also, the additional layer of a metallic conductor across the dielectric contributes to a loss of planarization in subsequent levels.
An alternative method published at page 118 of the 1984 IEDM Proceedings uses additional patterned silicon to provide conductive silicide regions extending over the field oxide as desired. A layer of titanium is deposited over the substrate and, prior to the direct reaction of the titanium with the underlying silicon to form the silicide, a thin layer of silicon is patterned over the titanium metal to define an interconnect extending over a silicon dioxide region separating the two regions to be interconnected. Where this silicon layer remains, a silicide is formed during the reaction process which extends over the oxides. This method requires the deposition and patterning of the additional layer of silicon to define the local interconnection. In addition, the resulting silicide strap provides a conduit through which typical n-type dopants such as phosphorus can diffuse, since titanium silicide is a very poor diffusion barrier to conventional semiconductor dopants. If a silicide strap is used to connect n-type regions to p-type regions, for example n-doped polysilicon to p-type diffusion, subsequent processing must be done at relatively low temperatures to minimize the counterdoping of the p-type region with the type dopant through the silicide interconnect.
Another known method uses molybdenum metal as a local interconnect material. Molybdenum, however, also acts as a diffusion conduit through which phosphorus, used to dope n-type regions of the semiconductor device, can diffuse. The molybdenum interconnect therefore is not an effective local interconnect between n-type and p-type regions because the p-type regions can be undesirably counterdoped by the phosphorus diffusing through the molybdenum, similar to the silicide strap interconnect.
A further local interconnect method is set forth in U.S. Pat. No. 4,675,073 wherein the desired local interconnect is formed by patterning the residual titanium compound, for example titanium nitride, from the direct reaction forming titanium silicide cladding of the diffusions and polysilicon gates. The titanium nitride is sufficiently conductive so that it is useful to make local interconnections between neighboring regions. The disclosed process uses carbon tetrafluoride (CF4) as the reactant in a plasma etch to remove the undesired titanium nitride faster than titanium silicide. This plasma etch using carbon tetrafluoride etches titanium nitride or titanium oxide at approximately twice the rate it remove titanium silicide. This technique also etches silicon oxides at twice the rate and photoresist at five times the rate as it etches titanium nitride or titanium oxide. Additionally, products of the etching process include solids that tend to adhere to the etching device. This requires extra maintenance and cleanup time that is nonproductive.
A still further local interconnect method is set forth in U.S. Pat. Nos. 4,793,896 and 4,863,559 wherein, in accordance with a first feature, a local interconnect is formed on a semiconductor surface by providing a dielectric of a prefabricated integrated circuit which is covered with an electrically conductive chemical compound of a refractory metal, such a compound formed during the silicidation of the refractory metal at locations where it is in contact with tho underlying silicon semiconductor material. A patterned masking material is formed over this chemical compound layer to protect a specific portion thereof. A chlorine bearing agent is used to etch all of tho conductive chemical compound layer except that portion which is protected by the patterned masking material. The chlorine bearing agent etches the conductive chemical compound at a greater rate than the underlying silicide and the dielectric layer. The patterned masking material is removed to expose the protected portion of the electrically conductive material to form a local interconnect on the integrated circuit. In accordance with a second feature, a layer of titanium nitride is formed as a by-product of the formation of titanium silicide by direct reaction, the layer of titanium nitride being present over the titanium silicide layer as well as over insulators such as oxide. A plasma etch using carbon tetrachloride as the etchant is used to etch the titanium nitride anisotropically and selectively relative to the titanium due to the passivation of the titanium silicide surface by the carbon atoms of the carbon tetrachloride. Excess chlorine concentration may be reduced, further reducing the undesired etching of the titanium silicide by providing a consumable power electrode or by introducing chlorine scavenger gases into the reactor. The plasma is ignited by exposing the gases to a mercury/argon light source, thereby photodetaching electrons from the anions in the gas.
Despite the fact that the above described prior art local interconnect methods operate with varying degrees of success, it is always desirable to provide alternate methods for providing local interconnect systems.